An optical element (such as a phase plate or a deflection element such as a diffraction grating) employing liquid crystal is small in size and has high durability since it has no mechanical moving portion, and thus has attracted attention as an optical modulation element, and as mounted on an optical head device, it is performing a role to modulate (e.g. deflect or diffract) a laser beam at the time of writing information on an optical disk or reading information from an optical disk.
For example, at the time of reading information, linearly-polarized light emitted from a laser light source is transmitted through a deflecting element and then through a ¼ phase plate and arrives at the surface of an optical disk. The polarization direction of the outward linearly-polarized light is aligned in a direction not changed by the deflecting element, and the outward linearly-polarized light is linearly transmitted through the deflecting element and transformed by the ¼ phase plate into a circularly-polarized light. This circularly-polarized light is reflected on the recording surface and becomes a reversed circularly-polarized light, which is again transformed by the ¼ phase plate into a linearly-polarized light perpendicular to incident light. Such a returning light beam will have its traveling direction bent when it is again passed through the deflecting element, and arrives at a light receiving element.
Further, during reading or writing of information, if the optical disk undergoes plane wobbling or the like, the focus position of the beam spot will be displaced from the recording surface, and a servo mechanism will be required to detect and correct the displacement to let the beam spot follow a concavo-convex pit on the recording surface. Such a servo system for an optical disk is constructed so that the focus of a beam spot irradiated from a laser light source is adjusted on the recording surface, and then the track position is detected to let the beam spot follow the desired track. Further, it is also necessary to make sure that the laser beam reflected without hitting the pit on the recording surface will not return as it is to the light source. Such optical elements are not limited to optical pickup elements used for reading records on optical disks, but they are utilized also for imaging elements in application to projectors, etc. or communication devices in application to wavelength-tunable filters, etc.
In recent years, in order to increase the capacity of optical disks, it has been attempted to shorten the wavelength of a laser beam to be used for writing or reading of information and to further reduce the concavo-convex pit size on optical disks. At present, a laser beam having a wavelength of 780 nm is used for CD, and a laser beam having a wavelength of 660 nm is used for DVD. For optical recording media of next generation, use of a laser beam having a wavelength of from 300 to 450 nm is being studied. Accordingly, a liquid crystal element is desired which modulates a laser beam having a wavelength of from 300 to 450 nm (hereinafter referred to also as a blue laser beam).
As a material to obtain an optical element employing liquid crystal (hereinafter referred to also as a liquid crystal element), a polymer liquid crystal obtained by polymerizing a liquid crystal composition containing a compound represented by the following formula (1) (wherein Q is a 1,4-phenylene group or a trans-1,4-cyclohexylene group, and Z is an alkyl group) has, for example, been reported (Patent Document 1).

However, conventional materials such as polymer liquid crystals disclosed in Patent document 1 have had a problem that the durability against a blue laser beam is inadequate. Therefore, the present inventors have developed various polymerizable liquid crystal compounds (Patent Document 2) as materials having good durability against a blue laser beam. Further, e.g. PCT/JP2005/001839 and Japanese Patent Application No. 2005-301138 report on polymerizable liquid crystal compounds having good durability against a blue laser beam.
The polymerizable liquid crystal compounds disclosed in e.g. Patent Document 2, PCT/JP2005/001839 and Japanese Patent Application No. 2005-301138 are compounds which satisfy the properties required for the materials to be used for optical elements (such as a large value of refractive index anisotropy, low absorption of a laser beam and wavelength dispersion of refractive index) and which at the same time have good durability against a blue laser beam.
Further, an element employing a polymer liquid crystal is prepared via a polymerization step such as polymerization after injecting a polymerizable liquid crystal into a cell, or polymerization after applying a polymerizable liquid crystal on a substrate. Such polymerization is usually carried out by photopolymerization. In a case where a polymerizable liquid crystal is injected into a cell or it is applied on a substrate, it is necessary to heat the polymerizable liquid crystal, and in order to prevent thermal polymerization, it is common to add a polymerization inhibitor to the polymerizable liquid crystal. Particularly, injection of the polymerizable liquid crystal into a cell is usually carried out by injection under reduced pressure. Accordingly, the polymerization inhibitor is required to have a high boiling point in addition to a high polymerization inhibiting ability. As such a polymerization inhibitor, 2-n-dodecylphenol is, for example, used.
On the other hand, a liquid crystalline material mixture containing an additive selected from the group consisting of a light stabilizer, a heat stabilizer and/or an antioxidant, added to a liquid crystalline mixture, has been reported (Patent Document 3). Patent Document 3 discloses that a 2,2,6,6-tetramethylpyperidine derivative or an alkylated monophenol such as 2,6-di-t-butyl-4-methylphenol may be used as the component of the light stabilizer, the heat stabilizer and/or the antioxidant.
Patent Document 1: JP-A-10-195138
Patent Document 2: WO2005/014522
Patent Document 3: JP-A-2002-536529